Numerical Investigation on the Jet Characteristics and Combustion Process of an Active Prechamber Combustion System Fueled with Natural Gas

Abstract

An active prechamber turbulent ignition system is a forced ignition method for internal combustion engines fueled with low reactivity fuels, i.e., natural gas and gasoline, which could expand the lean-burn limit, promote flame propagation, and ensure cyclic stability. In the present study, the effects of charge concentration stratifications inside the prechamber on the jet characteristics and combustion process were numerically investigated using CONVERGE software coupled with a reduced methane mechanism by the coupling control of spark timing and prechamber global equivalence ratio. The results show that the jet characteristics and ignition mechanisms can be regulated by controlling the prechamber global equivalence ratio and spark timing. On the one hand, as the prechamber global equivalence ratio increases, the velocity of the jet increases firstly and then decreases, the temperature drops, and OH and CH₂O radicals are reduced, but the stable combustion intermediates, CO and H₂, are increased. Thus, the ignition mechanism changes from flame ignition (ignition by flame and reactive radicals) to jet ignition (ignition by hot combustion intermediates), and the ignition delay is shortened, but the combustion duration is extended, mainly due to more of the combustion intermediates, CO and H₂, downstream of the jet. On the other hand, as spark timing is advanced, the jet velocity and the mass of the OH and CH₂O radicals increase, which is conducive to flame ignition, and the ignition delay and combustion duration are reduced.

1. Introduction

Due to climate change and environmental issues, it is increasingly important to reduce the greenhouse gas emissions from the transportation section, which has been regarded as one of the most important sources of greenhouse gases. Natural gas is a conventional low carbon fuel, which almost eliminates the emissions of sulfur oxides and particulate matter. When releasing per unit energy, it produces at least 30% less carbon dioxide than conventional petroleum fuels [1]. However, the limited flame propagation speed of natural gas is one of the major issues for natural gas engine thermal efficiency improvement. In addition, ignition and combustion stability become another issue when lean combustion is used for efficiency improvement, leading to problems such as an increase in unburned hydrocarbons and cycle-to-cycle variability.

Many studies show that the turbulent jet ignition (TJI) system can be a valid solution to solve the above-mentioned problems in natural gas engines [2,3].

TJI usually features a small prechamber (1–2%) with several nozzles, the reactive gases or turbulent flames produced by the prechamber are injected from the nozzle into the main chamber to ignite the lean mixture [4,5]. A large number of reactive species and high intensity turbulence make the ignition energy a million times higher than a conventional spark plug [6]. The prechamber can be mainly divided into two types: the passive prechamber and active prechamber. For the former, the fuel in the prechamber comes from the main chamber, while the latter has an individual fuel supply system. Blaxill et al. [7,8,9] conducted extensive research on both active and passive prechambers to analyze the effects of TJI on the combustion performance of engines. Benajes et al. [10] compared the potential of a passive prechamber and active prechamber in a high compression ratio engine. The results indicate that both passive prechambers and active prechambers could improve the combustion stability and efficiency, but active prechamber systems show further potential on expanding the lean-burn limit. Jamrozik et al. [11,12,13] carried out experiments and simulation studies on active prechambers. They found the active prechamber combustion system ensures burning ultra-lean mixtures with λ up to 2.0, and the COV of IMEP was approximately 3%.

Many researchers have studied the effect of various factors on TJI process. Shah et al. [14] experimentally explored the effect of prechamber volume on the ignition characteristics, and the results showed that the prechamber with a larger volume could provide higher ignition energy. Bigalli et al. [15] numerically studied the effects of prechamber shape and nozzle diameter on the combustion characteristics and found the orifice size has the most significant effect. Gentz et al. [16] studied the effects of nozzle diameter and number on TJI ignition by performing combustion visualization. It was found that in a stoichiometric mixture, jets produced by a nozzle with a wider spatial distribution could promote the rapid development of combustion, and in a lean mixture, a small nozzle diameter with a high jet velocity is required to ignite. Thelen et al. [17] numerically investigated the effect of spark plug position inside the prechamber and observed that the spark plug being away from the nozzle provides better ignition performance. Wang et al. [18] conducted a numerical simulation analysis on the effect of initial temperature on jet characteristics. Their results showed that the initial ignition position moved significantly towards the exit of the nozzle with the increase in temperature. Ju et al. [19] analyzed the effect of excess air ratio and premixed pressure through experiments, and found that ambient pressure has a greater effect on the flame propagation than excess air ratio air as the initial pressure increases. Onofrio et al. [20] studied the effect of spark timing on engine performance and found that a delay of spark timing increases the combustion duration and deteriorates engine performance. Zheng et al. [21] investigated the effect of injection parameters on the development of turbulent jet flame in rapid compression machines. Their results showed the initial lambda of the prechamber has a great influence on the jet velocity, while the initial lambda of the main chamber affects the jet penetration distance.

Based on the above studies, it could be found that the structures and control parameters of the prechamber have significant effects on the jet characteristics and combustion process. However, the TJI process is complicated due to turbulence, heat, and chemistry [22]. In order to understand the physical and chemical mechanisms behind it, many researchers have investigated the internal control and influential mechanisms of the TJI process. Benekos et al. [23,24] conducted a two-dimensional numerical simulation study on the ignition and initial stage of jet ignition, and found that the turbulent jet creates an intense flow field with vortical structures, which promote the interaction between jet and unburned mixture. Allison et al. [25] found that local quenching of free radicals occurs when passing through the orifice, which would change the compositional structure of the jet. Toulson et al. [26] investigated the mechanism of jet ignition through experiments and numerical simulations. The results showed that the type and quantity of free radicals play a major role in the chemical characteristics of jet ignition, while heat and turbulence play a secondary role. Qin et al. [27] used direct numerical simulation to investigate the transient mixing and ignition mechanisms of the prechamber. It was found that OH, CH₂O, and HO₂ have important effects on ignition and flame propagation in the main chamber. Pan et al. [28] numerically investigated the combustion evolutions of turbulent jet ignition. It was found that the flame configuration in the main chamber changes from normal turbulent jet flame propagation to spherical flame propagation as the temperature increases. Biswas et al. [29] defined two ignition mechanisms by using simultaneous high-speed schlieren and OH radical chemiluminescence imaging, jet ignition, and flame ignition, with the major difference between these modes being whether the flame is quenched.

As mentioned above, the jet characteristics and ignition mechanisms are complex and affected by many factors. However, the above-mentioned studies were mainly focused on passive prechambers, and fundamental studies on the turbulent jet formation and ignition process of active prechambers are still urgently needed—for instance, the effects of concentration stratifications inside the prechamber on internal flame and jet development. Motivated by this, this work numerically investigated the effects of prechamber concentration stratifications on the jet characteristics and combustion process, which is controlled by the spark timing and prechamber equivalence ratio in a constant volume chamber with an active prechamber fueled with natural gas.

2. Numerical Method

2.1. Numerical Model

This work was conducted using the computational fluid dynamics (CFD) software, CONVERGE [30].

Table 1. Sub-models in the simulation.

Model Type	Model Name
Turbulence model	Large Eddy Simulation [32]
Combustion model	G-equation coupled with the chemical reaction mechanism [33]
Heat transfer model	O’Rourke and Amsden [34]
Spark model	Spherical energy source

shows the major sub-models in the simulation. Large Eddy Simulation (LES) is adopted to simulate the turbulence. In addition, a G-equation coupled with a chemical reaction mechanism is applied for the combustion process prediction. Methane is used as a single component surrogate of natural gas, and the methane mechanism used in the present simulation includes 40 species and 139 reactions, which has been extensively validated with experimental data from shock tubes [31]. The spark event was modeled as a 0.5 mm spherical energy source centered in the plug gap, initiating at the spark time.

Figure 1 shows the three-dimensional geometric structure of the computational domain, which contains a prechamber, main chamber, nozzle, spark plug, and natural gas injector. The prechamber has dimensions of L (length) × D (diameter) = 40 mm × 20 mm. The main chamber has dimensions of L (length) × D (diameter) = 120 mm × 100 mm. The prechamber-to-main chamber volume ratio is 0.0132. The nozzle diameter is 2 mm, and the height is 17 mm. The spark plug and natural gas injector are located at the top of the prechamber—the former is centered, and the latter is biased.

Based on the above geometric model, in order to ensure the accuracy and efficiency of the numerical simulation, the basic mesh size of the prechamber and nozzle was set as 0.5 mm, and the basic mesh size of the main chamber was set as 4 mm. During the calculation, the mesh was generated by using four level automatic mesh refinement. In the end, the minimum mesh of the whole model was 0.25 mm and the maximum number of meshes was 2 × 10⁶.

The initial temperature in both the prechamber and main chamber was 700 K, and the pressure was 4 MPa, which is similar to the representative conditions of an actual engine. In order to individually explore the turbulent jet characteristics and also the ignition process in the main chamber, either N₂ or lean CH₄/air mixture was adopted in the main chamber.

2.2. Modeling Validation

The experimental results from Biswas et al. [29] were used to validate the current model. The conditions of the validation cases are given in

Table 2. The conditions of the validation cases.

Parameters	Case 1	Case 2
Fuel	CH4	CH4
Test number in the literature [29]	1	20
Prechamber volume	100 mL	100 mL
Main chamber volume	10 L	10 L
Nozzle diameter	4.5 mm	4.5 mm
Initial temperature	500 K	500 K
Initial pressure	0.1 MPa	0.4 MPa
Prechamber equivalence ratio	1.0	1.0
Main chamber equivalence ratio	0.8	0.9

.

Figure 2 shows the comparison between the experimental results and simulation results. The area shown is part of the main chamber, where x = 0 and z = 0 is the center of the nozzle exit. The left figures are the OH chemiluminescence images in the experiments, and the right figures are the OH mass fraction distribution images in the simulations. The upper figures are the comparison results of case 1, and the bottom figures are the results of case 2. In case 1, OH radicals could not be detected at the nozzle exit as the jet appears. After some time, a large number of OH radicals could be observed at a few centimeters downstream of the nozzle. Whereas, in case 2, a large number of OH radicals were detected as soon as the jet entered the main chamber, which means the flame is not quenched. Obviously, the ignition mechanisms of the two cases are different: case 1 is jet ignition and case 2 is flame ignition. The former is driven by the hot combustion products generated by the prechamber, and the latter is characterized by a jet of wrinkled turbulent flames and reactive free radicals. In both cases, the simulation results are generally in good agreement with the experimental results, so it can be considered that the model can predict the shape of the jet and ignition position well.

2.3. Definition of Jet Characteristics

Jet characteristics play a key role in ignition mechanisms and flame propagation.

Table 3. The definition of the ignition jet characteristics.

Jet Characteristics	Definition
Mixing characteristics	The velocity of jet center at nozzle exit
	Penetration distance
Thermal characteristic	Jet temperature
Chemical characteristics	OH mass and distribution
	CH2O mass and distribution
	CO mass and distribution
	H2 mass and distribution

shows the definition of the jet characteristics in this work. The high-speed jet disturbs the gas in the main combustion chamber so that the jet is in full contact with the unburned mixture, which is called the mixing characteristic. In addition, the jet carries heat and combustion products into the main combustion chamber, and this is called the thermal characteristic and chemical characteristic. In this work, the velocity of the jet center at the nozzle exit and penetration distance were used to represent the mixing characteristics of the jet. Among the mixing characteristics, there is no unified definition of jet penetration distance at present. Therefore, this work used the method of specifying the critical temperature to define the penetration distance. According to the literature [35], a temperature isosurface of 1200 K distinguished the burned zone between the unburned zone, so this paper defined the maximum length of the jet temperature that is higher than 1200 K as the jet penetration distance. Regarding the thermal characteristic, the jet temperature was used to represent it. The combustion products in the jet include highly-reactive free radicals, combustion intermediates, and complete combustion products. According to the literature [27], the heat release rate is proportional to the product of the reactive radicals, OH and CH₂O, mass fraction, and the stable combustion intermediates, CO and H₂, in the jet contribute greatly to the heat release. Therefore, this paper analyzed these four key species to explore the chemical characteristics of the turbulent jet on combustion.

3. Results and Discussion

3.1. Effect of Spark Timing on Jet Characteristics

The concentration stratifications in the active prechamber are different at different spark timings, which affects the flame propagation in the prechamber and jet characteristics. The conditions for studying the effect of spark timing on jet characteristics are specified in

Table 4. The conditions for different spark timings.

Parameters	Value
Injection pressure	22 MPa
Injection temperature	370 K
Injection mass	12.7 mg
Injection timing	0 ms
Injection duration	1.38 ms
Prechamber global equivalence ratio (ϕpc)	1
Spark timing	3 ms	5 ms	7 ms

, wherein N₂ is specified in the main chamber.

Figure 3 shows the distribution of the equivalence ratio in the prechamber for different spark timings. With the delay of spark timing, fuel stratification is reduced and the mixture distribution becomes more homogeneous in the prechamber. As can be seen, when spark timing is 3 ms, a fuel-rich region exists near the spark plug, and as the spark timings retard, the spark plug is surrounded by an almost stoichiometric mixture. In addition, it is observed that fuel-rich regions are mainly located in the top and middle of the prechamber, wherein feasible ignition conditions are available around the spark plug.

Figure 4 depicts the pressure traces of the prechamber for different spark timings. The maximum pressure of the prechamber decreases with the delay in spark timing. However, when the spark timing is delayed later than 5 ms, the effects of the spark timing on concentration stratification in the prechamber, and consequently on the pressure traces of the prechamber, become weaker. The velocity of the jet center at the nozzle exit for different spark timings is shown in Figure 5. Before spark timing, methane injection causes a disturbance in the prechamber, and part of the mixture flows to the main chamber. After spark timing, the pressure of the prechamber rises and part of the unburned mixture is pushed into the main chamber, which is called the cold jet. As the burning progresses in the precombustion chamber, the hot jet containing the majority of the combustion products appears in the main combustion chamber. The jet velocity at the nozzle exit is related to the pressure of the prechamber. The greater the pressure difference between the prechamber and main chamber, the higher the jet velocity. Therefore, jet velocity increases with the advance of spark timing; however, when the spark timings are 5 ms and 7 ms, the maximum jet velocity differences are small.

Figure 6 shows the jet temperature distribution and the centerline temperature traces for different spark timings at 3 ms after the hot jet appeared. It can be seen that the spark timing has little influence on the temperature characteristics and the penetration distance of the jet. Within 40 mm below the nozzle exit, the jet temperature is almost stable and remains above 2000 K. Then, the jet temperature drops rapidly between z = 40 mm and z = 50 mm. At the downstream locations, z > 50 mm, the jet temperature oscillates continuously. In all cases, the penetration distance is around 60 mm.

Figure 7 shows the distribution of the major species in the jet for different spark timings at 3 ms after the hot jet appeared. The highly-reactive OH radicals carried by the jet exist close to the nozzle exit where the temperature is high. Certain CH₂O molecules can be observed at the edge of the jet with relatively lower temperatures, and high concentrations of CH₂O can be found at the bottom of the main chamber. In addition, the combustion intermediates, CO and H₂, are mainly located near the nozzle exit region. The major species mass traces of the jet for different spark timings are shown in Figure 8. As the hot jet enters the main chamber, OH, CH₂O, CO, and H₂ gradually increase in the main chamber, then these species gradually decrease due to chemical reactions taking place inside the jet plume. As analyzed above, the chemical characteristics of the jets vary greatly under different spark timings, and significantly higher concentrations of OH, CH₂O, CO, and H₂ can be observed in the jet with the advance of spark timing.

3.2. Effect of the Prechamber Global Equivalence Ratio on Jet Characteristics

In this section, the effects of the prechamber global equivalence ratio on jet characteristics are explored by varying the methane injection mass of the prechamber. The boundary conditions of the simulations are given in

Table 5. The conditions for different prechamber global equivalence ratios.

Parameters	Value
Injection pressure	22 MPa
Injection temperature	370 K
Injection mass	12.7 mg	15.5 mg	18.2 mg
Injection timing	0 ms
Injection duration	1.38 ms	1.67 ms	1.94 ms
Prechamber global equivalence ratio (ϕpc)	1	1.25	1.5
Spark timing	3 ms	3 ms	7 ms

, and there is no fuel in the main chamber.

From the above section, it is found that an earlier spark timing shows advantages with jet velocity and chemical characteristics for ignition and combustion control. Figure 9 shows the distribution of the equivalence ratio in the prechamber for different global equivalence ratio cases. It can be seen that a high fuel concentration region exists in the upper right of the prechamber at 3 ms when ϕ_pc =1.25, wherein the equivalence ratio near the spark plug is suitable for spark ignition. However, the mixture near the spark plugs is too rich to ignite at 3 ms and 5 ms when ϕ_pc =1.5. At 7 ms, the mixture is leaner near the spark plug and the equivalence ratio is less than 1.5, which favors spark ignition. Therefore, the spark timings for three different prechamber global equivalence ratios, i.e., 1.0, 1.25, and 1.5, are 3 ms, 5 ms and 7 ms, respectively.

Figure 10 depicts the pressure traces of the prechamber for different prechamber global equivalence ratio cases. As the prechamber global equivalence ratio increases from 1.0 to 1.5, the maximum pressure of the prechamber increases first and then decreases. As shown in Figure 9, most of the mixture in the prechamber is near the stoichiometric ratio when ϕ_pc = 1.25. As a result, the combustion rate is the fastest and results in the highest peak pressure. The velocity of the jet center at the nozzle exit for different prechamber global equivalence ratios are shown in Figure 11. The greater the pressure of the prechamber, the higher the jet velocity at the nozzle exit. Therefore, when the prechamber global equivalence ratio increases, the maximum jet velocity firstly increases and then decreases. In all cases, the maximum jet velocity at the nozzle exit exceeds 900 m/s.

Figure 12 shows the distribution of the jet temperature and centerline temperature traces for different prechamber global equivalence ratios at 3 ms after the hot jet appeared. When the prechamber global equivalence ratio increases, the jet temperature at the exit of nozzle decreases. When ϕ_pc = 1, the jet temperature at the exit of nozzle is about 2150 K, while this temperature drops to about 2000 K when ϕ_pc = 1.5. In addition, although the prechamber global equivalence ratio has a significant influence on jet temperature, the jet penetration distances are quite similar—around 60 mm in all cases.

Figure 13 shows the distribution of major species in the jet for different prechamber global equivalence ratios at 3 ms after the hot jet appeared. When the prechamber global equivalence ratio increases, the OH and CH₂O mass fraction in the jet decreases, while both the CO and H₂ mass fraction increases. When ϕ_pc = 1.25 and ϕ_pc = 1.5, there is very little or no OH and CH₂O radicals in the jet, but a large number of the combustion intermediates, CO and H₂, diffuses from the exit of the nozzle to the bottom of the main chamber.

3.3. Effect of the Jet Characteristics on Ignition and Combustion in the Main Chamber

In this section, the effects of various turbulent jet characteristics on ignition and combustion were studied by adding a CH₄/air mixture with an equivalence ratio of 0.5 into the main chamber.

The distribution of major species in the main chamber at 3 ms after the hot jet entered the main chamber is shown in Figure 14. For different spark timings, the ignition mechanisms were not significantly affected by the velocity and chemical characteristics. The jets all carry a large number of OH radicals. This indicates that the flame passes through the nozzle without extinction, and the flame and reactive radicals ignite the mixture in the main chamber, which belongs to flame ignition. Due to the lean mixture in the main chamber, flame ignition does not give full play to its advantage of quickly igniting the surrounding mixture. For different prechamber global equivalence ratios, the differences in the velocity, temperature, and chemical characteristics have an important influence on the ignition mechanisms, among which the chemical characteristics play a key role. Comparing Figure 13 with Figure 14, the jet carries little or no OH and CH₂O radicals when ϕ_pc = 1.25 and ϕ_pc = 1.5. At the initial stage of ignition, the stable combustion intermediates, CO and H₂, in the main chamber decrease, while the OH and CH₂O radicals in the downstream of the jets increase greatly. This indicates that the hot combustion intermediates, CO and H₂, are oxidized in the downstream of the jets, thus igniting the surrounding mixture, which belongs to jet ignition. Therefore, as the prechamber global equivalence ratio increased, the OH and CH₂O reactive radicals in the jet decrease, and while the combustion intermediates, CO and H₂, increase, the ignition mechanisms change from flame ignition to jet ignition.

Figure 15 shows the histograms of the ignition delay and combustion duration for different spark timings and prechamber global equivalence ratios. The ignition delay represents the time duration from the spark ignition to when the heat release reaches 10% of the cumulated heat release in the main chamber, and the combustion duration represents the time duration from when the heat release reaches 10% of the cumulated heat release in the main chamber to 90%. It is seen that with the advance of spark timing, the ignition delay and combustion duration are shortened. On the one hand, with the advance of spark timing, the jet velocity is higher, and the jet disturbs the main chamber more intensely, which is conducive to the transfer of heat and the mixing of chemical components. On the other hand, with the advance of spark timing, there are more reactive free radicals and important combustion intermediates in the jet, which promote the chemical reaction and improve the ignition ability. With the increase of the prechamber global equivalence ratio, the ignition delay is shortened, but the combustion duration increases. It is mainly because the hot combustion intermediates, CO and H₂, that are downstream of the jet increase and ignite the lean mixture faster, but the ignition initiation location moves towards the bottom of the main chamber.

4. Conclusions

In this paper, a numerical investigation was conducted to explore the effects of prechamber concentration stratifications on the jet characteristics and combustion process in a turbulent jet ignition combustion engine based on a constant volume chamber, which is controlled by the spark timing and fuel mass (CH₄) inside the prechamber. The main conclusions of this present study are summarized as follows:

(1)

The sparking timing has a great impact on the jet velocity and chemical characteristics, while it has little impact on the thermal characteristics and penetration distance. With the earlier spark timing, the jet velocity is higher, and the jet contains more reactive chemical components, such as OH and CH₂O radicals.

(2)

The prechamber global equivalence ratio has a great impact on the jet velocity, temperature, and chemical characteristics, while it has almost no impact on the penetration distance. With the increase of the prechamber global equivalence ratio, the velocity of the jet increases firstly and then decreases, the temperature drops, and the OH and CH₂O free radicals are reduced, but the combustion intermediates, CO and H₂, increase.

(3)

Two different ignition mechanisms can be identified in the main chamber, i.e., flame ignition (ignition by flame and reactive free radicals) and jet ignition (ignition by hot combustion intermediates). As the prechamber global equivalence ratio increases, the ignition mechanism changes from flame ignition to jet ignition, and the ignition delay is shortened, but the combustion duration is extended, mainly due to an increase in hot combustion intermediates, CO and H₂, downstream of the jet.

(4)

The differences in the jet characteristics caused by spark timings have a limited effect on the ignition mechanism in the main chamber. Furthermore, the ignition mechanisms for different spark timings are all flame ignition. However, a higher jet velocity and more reactive intermediate species are beneficial to reduce the ignition delay and combustion duration with an earlier spark timing.